The most important factor for exhibiting the inherent strength of a carbon fiber in composite materials is a high interfacial bond strength between the fiber and matrix resin. Particularly, with increasingly higher tensile strength of carbon fibers, the energy released at fracture increases and as a result a high interfacial bond strength corresponding thereto becomes necessary. Further, a high carbonization temperature is necessary to obtain high modulus carbon fibers, as a result making the surface of the carbon fibers inactive. Consequently, it becomes necessary for higher performance carbon fibers to have the controlled interfacial bond strength.
To meet the above-mentioned necessities, it is a common practice to activate the surface of carbon fibers, as proposed for example in Japanese Patent Application Kokoku (Post-Exam Publn) No. 20033/80, by introducing functional groups with various surface treatments such as electrolytic oxidation or gas-phase oxidation, or by increasing the specific surface caused by physical etching. As carbon fibers of the higher performance are used, such surface treatments become more complex.
In general, the optimum level of surface treatment is determined based on some relationship between the surface treatment conditions and the mechanical properties of composites. However, even when the surface treatment conditions are kept constant, the surface state of the fiber can vary due to unexpected external disturbances such as variation of starting fiber and carbonization conditions. Consequently, there is always the danger of variation of composite properties.
Accordingly, it is not sufficient for controlling the properties of composite merely to keep the surface treatment conditions constant. Basically, it is necessary to control the surface state always to a constant condition.
Physical and chemical surface characteristics of carbon fibers are affected by surface treatment and as a result exert a great influence on the composite properties. In order to know clearly the surface treatment levels optimum for mechanical properties of a composite, it is necessary to grasp fully the surface state. Although there have been many examples of surface state analysis for carbon fibers such as measurement of the specific surface area by gas adsorption or analysis of functional groups by titration, ESCA and the like, they are no more than analytical studies in research stage, so these techniques are practically incapable of usage in industrial quality control.
In the prior art, no method was known for controlling surface treatment levels directly by measuring surface characteristics (as opposed to just the surface area) of carbon fibers. There was simply no available measuring method exhibiting a high enough correlation with the mechanical properties of the carbon fiber, and which could also be adapted to continuous measurement during carbon fiber manufacture. Two known measuring methods of surface characteristics are ESCA (EXPS) (electron spectroscopy for chemical analysis) and the BET (Brunauer-Emmett-Teller) method (for surface area measurement), but it is impossible to incorporate these methods into a process for carbonization of carbon fibers. The present invention makes this possible by applying a measuring method based on a new observation correlating surface characteristics of the fiber with certain electrochemical measurements.